The invention relates to a method of preparing an oriented cross-linked polymer, wherein a polymerizable liquid crystal composition, comprising at least a liquid crystal monomer, is oriented and subsequently polymerized. The invention also relates to liquid crystal compositions suitable for use in such a method and to cross-linked polymers obtainable by using such a method. The invention further relates to optical elements comprising cross-linked polymers obtainable by using such a method.
Oriented cross-linked polymers are useful materials, inter alia, for lighting and display applications, because they are optically anisotropic. Optically anisotropic materials can be used to manipulate the characteristics of light beams, such as polarizable, direction, intensity, color and splitting. Optical elements which can be made thereof are, for example, (polarizing) beam splitters, retardation plates, (reflective) polarizers and compensation foils.
A method of the type mentioned in the opening paragraph is described in U.S. Pat. No. Specification 5,042,925. In the known method, a uniaxially oriented liquid-crystal diacrylate monomer is photopolymerized using a photoinitiator and UV radiation so as to obtain a layer of a uniaxially oriented polymeric network or, in other words, a cross-linked polymer. The polymer is optically anisotropic, its birefringence being approximately 0.12.
A disadvantage of said known method is that it has limited means to adjust the optical properties, such as the (extra)ordinary refractive index or the birefringence, to desired values. Adjustment of the birefringence is achieved by adjustment of the temperature at which the polymerization is performed. In this way, only a small range of birefringence values is reliably accessible. In particular, low degrees of optical anisotropy are difficult to obtain since this requires polymerizing at a temperature close to the clearing temperature (that is the temperature at which the transition between the liquid crystalline and isotropic phases occurs) of the liquid-crystal composition. At such critical temperatures, the optical properties are rather sensitive to small changes in temperature, the orientation is easily lost and undesirable biphasic structures easily develop.